1. Field of the Invention
The present invention generally relates to a prosthetic device for tissue repair, and, more particularly, to a surgical silk mesh device employing a stable knit structure.
2. Description of Related Art
Surgical mesh initially used for hernia and abdominal wall defects are now being used for other types of tissue repair, such as rotator cuff repair, pelvic floor dysfunction, and reconstructive or cosmetic surgeries. It is projected that in 2010, there will be more than 8 million hernia procedures, 800,000 rotator cuff repairs, 3 million pelvic prolapse repairs, 600,000 urinary incontinence repairs, and 1.5 million reconstructive or aesthetic plastic surgeries. Most of these procedures will likely employ implantable surgical mesh devices currently on the market, including: Bard Mesh (polypropylene) by C. R. Bard; Dexon (polyglycolic acid) by Synecture/US Surgical; Gore-Tex (polytetrafluoroethylene) by W.L. Gore; Prolene (polypropylene), Prolene Soft (polypropylene), Mersilene Mesh (polyester), Gynemesh (polypropylene), Vicryl Knitted Mesh (polyglactin 910), TVT (polypropylene) by Ethicon; Sparc tape (polypropylene) by American Medical Systems; and IVS tape (polypropylene) by TYCO Healthcare International.
Surgical mesh devices are typically biocompatible and may be formed from bioresorbable materials and/or non-bioresorbable materials. For example, polypropylene, polyester, and polytetrafluoroethylene (PTFE) are biocompatible and non-bioresorbable, while polyglactin 910 and polyglycolic acid are biocompatible and bioresorbable.
Though current surgical mesh devices may be formed from different materials, they have various similar physical and mechanical characteristics beneficial for tissue repair. However, despite the benefits provided by current surgical mesh devices, their use may be accompanied by a variety of complications. Such complications, for example, may include scar encapsulation and tissue erosion, persistent infection, pain, and difficulties associated with revision surgery. In addition, the use of an absorbable material may result in reoccurrence due to rapid resorption of the implant material and loss of strength.
Although polypropylene monofilament may be a highly regarded material for surgical mesh devices, polypropylene mesh devices can induce intense scar formations and create a chronic foreign body reaction with the formation of a fibrous capsule, even years after implantation. Minor complaints of seromas, discomfort, and decreased wall mobility are frequent and observed in about half of the patients implanted with polypropylene mesh devices. Moreover, polypropylene generally cannot be placed next to the bowel due to the propensity of adhesion formation.
Although the use of multifilament polyester may improve conformity with the abdominal wall, it is also associated with a variety of disadvantages. For example, higher incidences of infection, enterocutaneous fistula formation, and small bowel obstruction have been reported with the use of multifilament polyester compared to other materials. Indeed, the small interstices of the multifilament yarn make it more susceptible to the occurrence of infection, and thus multifilament polyester is not commonly used within the United States.
The use of polytetrafluoroethylene (PTFE) may be advantageous in minimizing adhesions to the bowel. However, the host tissue encapsulates the PTFE mesh, resulting in weak in-growth in the abdominal wall and weaker hernia repair. This material, though not a good mesh material on its own, has found its place as an adhesion barrier.
Absorbable materials, such as Vicryl and Dexon, used for hernia repair have the advantage of being placed in direct contact with the bowel without adhesion or fistula formation. A study has observed that Vicryl has comparable burst strength to nonabsorbable mesh at three weeks but is significantly weaker at twelve weeks due to a quick absorption rate. Meanwhile, the same study observed that Dexon has more in-growth at twelve weeks with less absorption of the mesh. The concern with absorbable meshes is that the rate of absorption is variable, possibly leading to hernia recurrence if the proper amount of new tissue is not there to withstand the physiologic stresses placed on the hernia defect.
A significant characteristic of a biomaterial is its porosity, because porosity is the main determinant for tissue reaction. Pore sizes of >500-600 μm permit in-growth of soft tissue; pore sizes of >200-300 μm favor neo-vascularisation and allow mono-morphological restitution of bony defects; pore sizes of <200 μm are considered to be almost watertight, hindering liquid circulation at physiological pressures; and pores of <100 μm only lead to in-growth of single cell types instead of building new tissues. Finally, a pore size of <10 μm hinders any in-growth and increases the chance of infection, sinus tract formation, and encapsulation of the mesh. Bacteria averaging 1 μm in size can hide in the small interstices of the mesh and proliferate while protected from neutrophilic granulocytes averaging 10-15 μm.
Other important physical characteristics for surgical mesh devices include thickness, burst strength, and material stiffness. The thickness of surgical mesh devices vary according to the particular repair procedure. For example, current surgical mesh device hernia, pelvic floor dysfunction, and reconstructive/cosmetic procedures range in thickness from approximately 0.635 mm to 1.1 mm. For rotator cuff repair, a thickness of 0.4 mm to 5 mm is typically employed.
Intra-abdominal pressures of 10-16 N, with a mean distension of 11-32% results in the need for a surgical mesh with a burst strength that can resist the stress of the inner abdomen before healthy tissue comes into being.
Material stiffness is an important mechanical characteristic for surgical mesh, especially when used for pelvic floor dysfunction, because material stiffness has been associated with the likelihood of tissue erosion. Surgical mesh devices formed from TVT, IVS, Mersilene, Prolene, Gynemesh, Sparc tape, for example, currently have an ultimate tensile strength (UTS) that exceeds the forces exerted by intra-abdominal pressures of 10-16 N. With the low force in the abdomen, the initial stiffness of the material is an important consideration. Moreover, the stiffness may exhibit non-linear behavior most likely due to changes in the fabric structure, e.g., unraveling of the knit, weave, etc. A surgical mesh device of lesser stiffness may help reduce tissue erosion and may conform to the contours of the body more effectively.